Any Russian aggression in Ukraine would trigger "serious consequences", US Secretary of State Antony Blinken said on Tuesday as Nato foreign ministers held talks on Moscow's unexplained troop build-up.
Mr Blinken said the movements on Ukraine's eastern border were unusual and that any escalation would be of "great concern to the United States".
He was expected to brief his fellow foreign ministers at the summit in Riga, Latvia, on US intelligence regarding Ukraine.
Nato says Russia has combat-ready troops on the Ukrainian border, seven years after it annexed Crimea in a move not recognised by most of the international community.
"There will be a high price to pay for Russia if they once again use force against the independence of the nation Ukraine," Nato Secretary General Jens Stoltenberg said.
Russia denies any intention to invade Ukraine, its ex-Soviet neighbour.
Britain urged Nato members to support Ukraine's defences and told Moscow that attacking the country would be a mistake.
But Vladimir Putin, the Russian President, said the Kremlin would move to defend itself if missiles were positioned within range of Moscow, crossing a "red line".
"If some kind of strike systems appear on the territory of Ukraine, the flight time to Moscow will be 7-10 minutes, and five minutes in the case of a hypersonic weapon being deployed. Just imagine," Mr Putin said.
"What are we to do in such a scenario? We will have to then create something similar in relation to those who threaten us in that way. And we can do that now."
UK Foreign Secretary Liz Truss dismissed Mr Putin's suggestion that Russia was being provoked by Nato.
"We have seen this playbook from the Kremlin before when Russia falsely claimed its illegal annexation of Crimea was a response to Nato aggression," she said.
"Nato is an alliance forged on the principle of defence, not provocation. Any suggestion that Nato is provoking the Russians is clearly false."
Ukrainian President Volodymyr Zelensky said on Friday that intelligence services had uncovered plans for a Russian-backed coup in Kiev.
Mr Blinken said Russia often combined troop movements with internal efforts to destabilise a country. "That’s part of the playbook and we’re looking at it very closely," he said.
Poland, a Nato member, suspects Russia of playing a role in the border crisis in Belarus, an ally of Moscow. Belarus on Monday announced joint military drills with Russia.
Alexander Lukashenko, the President of Belarus, said he would ask Moscow to provide nuclear weapons if Poland were similarly equipped by Nato.
"We are ready for this on the territory of Belarus," he told Russia's RIA news agency.
